1. Field of the Invention
The present invention relates to a battery state monitoring circuit capable of controlling a charging/discharging operation of a secondary battery and a battery device using the circuit.
2. Description of the Related Art
A power supply device shown in a circuit block diagram of FIG. 2 has been known as a conventional battery device composed of a secondary battery. That is, a secondary battery 201 is connected with an external terminal −V0205 or +V0204 through a switch circuit 203 which is a current limiting unit. Further, a battery state monitoring circuit 202 is connected in parallel with the secondary battery 201. The battery state monitoring circuit 202 has a function for detecting a voltage and a current of the secondary battery 201. In any of: an over-charge state in which the secondary battery 201 has a voltage value higher than a predetermined voltage value; an over-discharge state in which the secondary battery 201 has a voltage value lower than a predetermined voltage value; and an over-current state in which the current flowing in the switch circuit 203 exceeds a predetermined current value with the result that the external terminal −V0205 reaches a certain voltage, a charge/discharge inhibition signal is outputted from the battery state monitoring circuit 202 such that the switch circuit 203 can be turned off to suspend a charge current or a discharge current. Here, the charge/discharge inhibition signal is outputted by a necessary delay time, thereby preventing a malfunction resulting from a temporal noise (for example, see JP 04-075430 A (FIGS. 1 and 2)).
In the conventional power supply device, the malfunction resulting from the temporal noise can be prevented. However, when a continuous noise enters the device, the above-mentioned states become hard to be detected, thereby shifting a detection voltage.
If the over-charge detection is hard to be effected to shift the detection voltage to a higher voltage, the secondary battery is over-charged, so that a safety margin is reduced. In addition, if the over-discharge detection is hard to be effected to shift the detection voltage to a lower voltage, the secondary battery is over-discharged, so that a battery life is shortened. Further, if the over-current detection is hard to be effected to shift the detection current to a higher current, a large amount of excessive current flows through the switch circuit 203, so that a life of the switch circuit 203 is shortened. Thus, in order to avoid the above-mentioned inconveniences, it is necessary for the conventional power supply device to set the respective detection voltages in a safety margin range.